In 1992-1994, UK testing of school performance showed that "a disproportionate share of the best schools, academically, are single-sex," both public and private (sectarian or secular).
Same-sex schools are inherently selective of the highest performing students.
Co-ed schools which are selective perform better, too.
The UK studies indicated that girls and boys up to age 16 perform equally in single-sex or co-ed academic settings.
But girls perform better from age 16 on in same-sex academic environments, than in co-ed settings.
This was attributed to girls' tendency toward "lack of self-confidence" and deference to boy's in the classroom, even in the presence of under-performing but uninhibited boys, who typically do not mature and begin to study diligently until after age 17 or 18.
Women at single-sex colleges are more likely to graduate and obtain advanced degrees than women at co-ed schools.
A highest-ranked UK school headmistress summed it up: "... 16-year-old girls work a lot harder than boys," especially where they "don't have to conform to gender stereotypes", "not just in science and maths, but in behaviour generally," when they "believe that they have a real chance of entering a career with their academic qualifications, and so they are working for them." An untested, anecdotally proffered advantage of same-sex schools is that students are less distracted from studies if emerging sexual-tensions are avoided at least in the classroom.
An untested, anecdotally asserted disadvantage is that same-sex schools "are a barrier to successful socialization of adolescents", and therefore overall success in life.
